Irradiation and Size Effects on Redox Reaction Mechanisms in Iron Oxides

The ability to understand and control the reaction mechanisms of iron (Fe) nanoparticles (NPs) has an important impact on their applications. In this paper, applying in situ transmission electron microscopy (TEM), we studied the irradiation- and size-dependent redox kinetics in Fe. (1) In the irradiation effect, weaker (intensity: ∼102 A/cm2) and stronger (intensity: ∼103–104 A/cm2) electron beam irradiations may facilitate the oxidation and reduction of Fe NPs, respectively. (2) In the size effect, Fe with a smaller size is first oxidized into FeO, while a large-sized Fe is directly oxidized into Fe3O4. The critical size is estimated to be ∼2.5 nm. Besides, both the atomistic investigations and theoretical calculations reveal the Fe/FeO and FeO/Fe3O4 phase boundary evolutions, which play a pivotal role in understanding the cation and anion migration pathways during the redox reactions. These results shed light on the redox mechanisms in Fe–O compounds and pave the way for improving their structural diversity and exploiting their potential applications.